1. Field of the Invention
The present invention pertains to a method and system for determining the friction pressure losses in fluids being pumped through generally tubular conduits, including non-Newtonian drilling and fracturing fluids used in the production of subterranean hydrocarbon deposit.
2. Background
In the production of oil and gas from subterranean deposits it is important to be able to determine the actual fluid pressure at a particular point in the wellbore which may be several thousand feet below the surface. In particular, it is important to know the so-called bottom hole pressure in an injection well or during fracturing of a formation with pumped fluids. There are also other operations where direct measurement of fluid pressure loss in a conduit is not easily obtainable at the desired point in the conduit.
In oil and gas well stimulation operations involving formation fracturing, large quantities of non-Newtonian fluids or generalized Newtonian fluids are pumped down the wellbore at relatively high velocities. The actual pressure of the fluid at the point of injection is the measured pressure at the wellhead plus the hydrostatic pressure of the fluid column minus the friction pressure loss between the point of measurement and the point of injection deep in the wellbore. So-called bottom hole pressure is important to determine in regard to determining the response of a formation to the treatment process and for purposes of conducting an analysis of the geometry of the fracture propagation.
Conventional techniques for determining fluid friction pressure losses in a wellbore include determining readily obtainable parameters such as the flow velocity and the geometry of the conduit through which the fluid is being pumped. These factors, together with laboratory measurement of flow indices for predetermined types of fluids, have been used to calculate predicted friction pressure losses in the wellbore before fracturing type stimulation techniques as well as other fluid pumping operations which require relatively accurate determination of pressure at a particular point in a conduit.
Although flow indices such as the consistency index (K') and the so-called Power Law index or flow behavior index (n') may be obtainable for a particular fluid composition from laboratory measurements, the characteristics of fluids actually used in an operation may vary substantially thereby resulting in relatively serious errors in friction pressure loss calculations. Recent developments in the provision of inline pipe viscometers have been of some assistance in determining friction pressure losses.
In oil and gas well hydraulic formation fracturing operations, in particular, and certain other operations involving the use of fluids which behave generally according to the so called Power Law or Ostwald-de Waele model, the fluid characteristics typically change on site during the operation. Accordingly, the use of predetermined consistency and flow behavior indices not only introduce serious pressure calculation errors, but the use of various measuring devices which are disposed directly in the flow line such as inline pipe viscometers is precluded by the presence of abrasives in the fluid such as sand proppants and the like which damage or destroy the measuring devices after only brief periods of use.
Accordingly, there has been an acute need to develop improved methods of determining fluid friction pressure losses which occur in subterranean formation fracturing operations and similar operations wherein direct pressure measurements at the desired points to be considered are not readily obtainable. It is to this end that the present invention has been developed which provides an improved process and system to provide for determining pressure losses in a flowing fluid stream due to pipe friction and other energy losses due to fluid flow through tubular conduits and the like, including subterranean wellbores.